1. Field of the Invention
The present invention generally relates to an apparatus and method for detection of ionizing radiation, particularly but not exclusively X-rays.
The invention is usable in a variety of fields including e.g. medical radiology, computerized tomography (CT), microscopy, and non-destructive testing.
2. Background Information
Scintillator based detection systems are widely used for high-resolution imaging of gamma and x-rays. Such imaging systems use the detected radiation to produce a signal, which can be used to operate a visual display, such as a cathode ray tube.
One example of such an imaging system is the Anger camera, which is commonly used in medical diagnostic procedures. In the Anger camera, incident radiation passes through a collimator before striking a scintillator layer. Light generated by the interaction of the incident radiation and the scintillator material then spreads out through an underlying light guide until it strikes an array of photomultipliers. The intensity of the light striking the individual photomultipliers varies dependent on the distance of the photomultiplier from the point where the incident radiation interacted with the scintillator to produce the initial light burst. A resistor network electrically determines the point of the radiation impact on the array based upon the magnitude of the respective electrical output of the photomultiplier devices in the array; summing the electrical output signals provides a measure of the energy level of the initial incident radiation. The low efficiency of the light guide and the poor photoemission conversion of the photomultipliers result in significant statistical fluctuation of collected light photons, which causes degraded spatial and energy level resolution. Additionally, Anger cameras have relatively low count rates as every incident gamma ray that interacts with the scintillator material results in substantially the entire array being rendered non responsive until the light generated from the earlier interaction has diminished.
Another common prior art device is known as an image intensifier gamma camera. In such devices, the scintillator is shaped to be tightly coupled to the transparent window of a large field of view image intensifier tube, which discharges photoelectron energy packets in response to the light signal from the scintillator. The photoelectron packets are accelerated and focused onto a cathodoluminescent phosphor deposited on a fiber optic output plate, generating additional light photon bursts. Multiple image intensifier stages can be coupled together to further amplify the signal. The final burst of photoelectrons will generate charges on a resistive charge divider from which the center of gravity of the pulse is reconstructed. Image intensifier cameras have substantial weight, size and expense, which inhibit their practical use for many applications.
A solid-state radiation detector is disclosed in U.S. Pat. No. 5,144,141. In this detector, radiation incident on the detector passes through a collimator and strikes a scintillator, which is divided into a plurality of scintillator elements arranged in rows and columns. An array of internal gain photodetectors divided into rows and columns are optically connected to the scintillator elements. Each photodetector is electrically coupled to a respective detect and hold circuit which amplifies and stores the pulse generated by the photodetector; the stored pulses are sampled via a multiplexed switching arrangement to allow the stored signal from each detect and hold circuit to be processed to produce a digitized imaging signal, which corresponds to the energy level of, and location on the array of, the detected incident radiation. The digitized imaging signal is supplied to display memory and analysis equipment for the device.
Particularly for radiation imagers employed in medical procedures, in which it is desired to expose the patient to the minimum amount of ionizing radiation as possible, it is important that the imaging device be sensitive to low levels of radiation while still being able to discriminate against background radiation. Solid-state detectors have a somewhat limited amplification due to losses, and thus in some application they do not possess sufficiently high signal-to-noise ratios.
Improved spatial resolution requires the use of a large number of photodetectors and a scintillator system, which generates light photons only in the scintillator segment in which the incident radiation was absorbed. The use of a larger number of photodetectors in a large array or to increase the resolution of the device rapidly results in very complex and expensive apparatus.
Further, photodetectors are sensitive to direct irradiation by the incident X-rays and hence measures have to be taken in order to prevent the incident radiation from reaching the photodetectors.
Additionally, solid-state radiation detectors have a limited speed. They normally require long integration times in the electronics, several microseconds, in order to capture a large fraction of the signal and keep down the noise level. This prevents them in most applications to be used for single photon detection. Additionally the noise level in solid state detectors is normally too high to be able to detect single photons.
Accordingly, the present invention to provide an apparatus and method for detection of ionizing radiation, particularly X-rays, which provide for an effective amplification and having high signal-to-noise ratios.
The invention provides such detection apparatus and method, which provide for high sensitivity, and can thus operate at very low X-ray fluxes.
The present invention provides such detection apparatus and method, having detection elements, which are insensitive to direct irradiation by the ionizing radiation.
The present invention provides a detection apparatus and method, which are capable of detecting and resolving single light photons emitted from a scintillator by a single X-ray. This allows a more accurate determination of the X-ray energy than conventional integrating techniques.
The present invention provides such detection apparatus and method, which are effective, fast, accurate, reliable, and of low cost.
The present invention, is attained by apparatus and methods as claimed in the appended claims.
By employing avalanche amplification of electrons released from the photocathode of the detection apparatus a particularly sensitive apparatus and method are achieved, which provide for the employment of extremely low doses of radiation, still obtaining signal levels high enough for construction of two-dimensional images exhibiting very low noise levels.
The inventive detector is not very sensitive to magnetic fields.
Yet a further advantage of the invention is that it provides for the manufacture and use of sensitive large-area detectors to a low cost.